Generally, a non-volatile memory retains data even if power is switched off. Accordingly, the non-volatile memory is mainly used for data storage of a PC bias, a set-top box, a printer, and a network server. Recently, the non-volatile memory is used in a digital camera and a portable phone.
Among the non-volatile memories, an electrically erasable programmable read-only memory (EEPROM)-type flash memory device erases data of a memory cell all at once or according to a sector unit. In programming, the flash memory device generates channel hot electrons from a drain and charges the electrons in a floating gate, thereby increasing a threshold voltage of a cell transistor.
In erasing, the flash memory device generates a high voltage between a source/substrate and the floating gate to emit electrons from the floating gate such that the threshold voltage of the cell transistor can be lowered.
Recently, as high integration is rapidly performed, the reduction of a cell size is required. An EEPROM tunnel oxide cell (ETOX) currently has the smallest cell size and has an array structure in which two polysilicon patterns are stacked. A lower polysilicon pattern of the two polysilicon patterns may serve as a floating gate, and an upper polysilicon pattern may serve as a select gate.
In this structure, over-erase may occur.
In order to solve the over-erase, a cell having a split gate structure is used.
Although the over-erase has been solved by employing the split gate structure, the length of a gate of a transistor serving as the select gate may be varied by a misalignment degree, effecting an erase state due to the misalignment between the floating gate and the control gate. Accordingly, a difference is made in current distribution. In other words, uniform characteristics are not represented between cells.
If a common source structure is used to reduce a cell size, the current of even/odd word lines arranged at each side of a source line may be varied.
As described above, when forming a gate, an erase current and a coupling ratio have variations due to a photo process and overlay variation, so that uniform erase characteristics cannot be ensured between cells, and reliability of the flash memory device is degraded.